1. Field of the Invention
The present invention relates to a security element in the form of a thread or strip to be embedded in security documents such as bank notes, checks, bonds or the like, having characters, patterns, etc . . . that are readable by transmitted light to the naked eye and/or by machine, and to methods of producing such security documents.
2. Description of the Related Technology
It has been known for some time to protect bank notes and other money-value papers such as cheeks, shares, travelers checks, check cards and credit cards, but also passports and identity cards, by adding socalled security threads. Papers of this kind shall be referred to in the following as security papers. The security threads used for this purpose are made of a great variety of materials in the form of fine strips which are added in widths of 0.4 to 2 mm to the paper sheet during its manufacture. They are primarily produced from plastic films coated with metal foils, printed, colored or provided e.g. with pigmentlike substances. Furthermore, it is also known to give these security threads magnetic, fluorescent, X-ray absorbent and other properties by applying appropriate substances.
It has proven particularly advantageous to use security threads with microprints, whereby this microprint may contain information that is readable by the naked eye or else only by machine.
With respect to protection from forgery, security threads with metallically reflecting surfaces have proven particularly useful, since such threads introduced inside the paper are almost invisible by incident light, as the light rays passing through the upper layer of paper are completely reflected by the, metallic surface and leave the paper diffusely scattered. By transmitted light, however, such threads appear as black strips which are clearly distinct from their surroundings. Threads of this kind cannot be imitated by printing on the paper. On the other hand, they show the disadvantage that a corresponding impression can be incorrectly aroused by introducing thin aluminum foils between two layers of paper. It has therefore been proposed to provide such aluminized security threads with microwriting (German laid open print no. 14 46 851). However, this has proven to be of little use in practice since this writing cannot be detected on the otherwise opaque security thread by transmitted light, and can only be detected with great difficulty by incident light. It is usually necessary to make the paper transparent with chemical means at least for the time of testing. The execution of such microwriting in special, e.g. fluorescent, colors has proven to be of little advantage in the same way and for the same reasons.
German xe2x80x9cAuslegeschriftxe2x80x9d no. 22 05 428 discloses a security thread designed as a metal strip and having a machine readable and/or visually readable coding applied by laser. The writing on this thread, that may possibly contain alphanumeric characters, consists of perforations, whereby the diameter of the holes or xe2x80x9cline widthxe2x80x9d is to be small compared to the thickness of the metal strip in order to impede imitation.
Due to their extremely small line width, the characters of this security thread are detectable as poorly visible lighter areas on the dark background. Furthermore, such writing is relatively troublesome to produce, since very powerful lasers are required to provide the perforations in the metal thread. The perforation of the security thread material is so time-consuming that this method cannot be used for producing large amounts (thousands of kilometers) simply for reasons of time and thus of costs.
U.S. Pat. No. 4,652,015 also discloses a security thread to be used for bank notes and the like, on which metallically shining microcharacters are provided on clear transparent film material. The security thread and microcharacters located thereon as described in this patent are not detectable by incident light. However, by transmitted light solely the characters are detectable as sharp contours since the security thread itself is of transparent design. This security thread is produced by metalizing a transparent film over a large surface with a thin aluminum layer, printing the microcharacters on this layer using an acidproof ink and then etching away the unprinted areas, whereupon the characters are left on a transparent base.
By transmitted light an observer thus sees only writing that runs through the bank note. However, a disadvantage is that this writing is difficult to find due to the small size of the characters, its embedding in the paper pulp and a printed pattern that may be superimposed. The thread as such cannot be detected by the naked eye and cannot be felt as an uneven formation on the surface of the paper due to the extremely thin design required here. These detection features that normally characterize a security thread are thus no longer present in the known thread.
The invention is based on the problem of providing a security thread with characters and patterns that are readable to the naked eye and/or machine, whereby both the thread structure and the characters and patterns are clearly recognizable when the thread is embedded in a document, and whose visual impression and protection from forgery are improved.
This problem is solved by the features stated in the characterizing part of the main claim. Developments of the invention and methods for producing such security elements are the subject of subclaims and independent claims.
In a preferred embodiment, the security thread consists of a transparent plastic film provided on one side with a metal coating. Characters are introduced into this reflecting metal coating, that is opaque by transmitted light, by locally removing the coating material. In addition to this metal coating, the thread is provided with a coat of color which may extend over the entire thread surface. The coloring agents used have no opaque effect and are preferably glazing printing inks in various colors and tones. However, the plastic film may also be colored with appropriate coloring agents in such a way that its transparency is retained in a partial region of the visible spectrum.
When such a thread is regarded by transmitted light after it is embedded in paper or only white translucent plastic material, as is sometimes used for producing identity cards or credit cards, the thread is detectable quite readily as a dark strip in the document and the characters and patterns are distinct as light, colored areas compared to their direct surroundings, the opaque thread coating, and additionally compared to the wider surroundings, the white paper or plastic material. The thread is thus very easy to find in the document, and the characters are clearly recognizable due to their contrasting effect with the surroundings and can be read at any time without aids if their size is appropriate.
While the characters thus appear by transmitted light as colored characters compared to the dark or white background, the thread is invisible or only barely visible by incident light due to the greatly reflecting metal coating and the use of glazing or nonopaque coloring agents. It can therefore not be imitated by an external print, which is inevitably clearly visible both by incident light and by transmitted light.
Suitable opaque coating materials are not only metal layers but also nonmetallic layers that contrast with their surroundings in terms of color and/or gray tones when viewed by transmitted light, such as opaque, preferably white, layers of color, metallically shining layers such as titanium nitride, interference layers such as those disclosed e.g. in U.S. Pat. No. 3,858,977.
By using luminescent colors, one can further improve the visual impression of this thread and make it more effective. The luminescent colors can either be colorless in the unexcited state or have a body color that preferably differs from the color of the emission light. The characters or patterns then appear in color or change their color only in the excitation light when the thread is regarded e.g. in UV light. Several luminescent substances emitting in different colors increase both the possibilities of design and the protection from forgery, since accurate imitation can only be performed by analyzing each individual luminescent substance. This analysis can easily be impaired further by printing the different luminescent colors in a mixed or overlapping fashion.
Diffraction grids or holograms can also be used to obtain a great variety of color effects. The diffraction structures are present e.g. in the form of volume or embossed holograms directly as embossing in the carrier material of the plastic thread or in an additional layer.
The reflecting metal layer existing in the case of reflection holograms or grids is interrupted here preferably by one of the methods stated below, to produce writing or a pattern visible by transmitted light.
Such a thread is preferably embedded in the document in such a way that it is directly visible or appears on the surface at least in some places. Methods for embedding it in the paper of value so as to form a window in the security thread area are known e.g. from German laid open print no. 36 01 114. With a thread embedded in this way, primarily the hologram diffraction grid or reflection pattern is thus visible in the reflection in the window area, while the negative writing or pattern worked into the metal coating dominates when viewed by transmitted light.
To produce such security threads, one first vaporizes a thin aluminum layer over the entire surface of e.g. colored, printed and/or luminescent plastic films that are transparent in a partial region of the visible spectrum. The blank areas are then applied in this metal layer in the form of the desired characters and patterns by known methods (etching, spark erosion, etc.). The thread thus produced then shows the desired properties. If spark erosion (also known as electroerosion) is used, it is advantageous if the electrodes already have the form of the characters and patterns to be provided. In this way one can obtain extremely fine microcharacters of good quality.
According to a preferred production method, one resorts to the means basically known from printing technology for producing the characters and patterns, printing them on the metalized side of the film by known microprinting methods. However, one uses a printing ink that has e.g. thermoplastic properties, i.e. becomes soft and sticky at higher temperatures. If a film pretreated in this way is laminated by means of heat and pressure against a second untreated sheet of film, the two films adhere to each other in the area of the printed characters or patterns. When the cooled films are later separated, the areas corresponding to the characters or patterns are taken out of the aluminum coating. One thus obtains a metalized film material having characters or patterns in the form of transparent colored dots or lines in the otherwise opaque reflecting surface.
It is advantageous if the film for producing the security thread is first provided with a priming in the form of printing ink before the above-described method is carried out, and this prime coat is then metalized. In this case, one can apply the prime coat in the form of a colored and/or luminescent surface instead of using colored films.
In a further embodiment, this colored prime coat can also be executed in the form of a multicolored printed pattern, resulting e.g. in a random distribution of the coloring in the various characters. One can thus produce threads having microwriting that shimmers in many colors and appears light on a black background.
In a further embodiment, the film material can first be provided with the colorless prime coat, while the colored transparent layer of ink is applied to the opposite side of the film. This method offers advantages if different qualities of ink must be used for the prime coat and colored transparent layers. After subsequently metalizing the prime coat, one can proceed in the above-mentioned way.
The hot embossing method known as such can also be used advantageously for selectively metalizing colored or printed security threads. This method can be used advantageously in particular in connection with xe2x80x9clight collecting filmsxe2x80x9d as the film material. These films have the property of xe2x80x9ccollectingxe2x80x9d incident light and making it emerge in a certain color and only in edge areas or at irregularities in the surface. Since the hot embossing method involves embossing the characters or patterns into the film and these embossed structures constitute such irregularities, the characters appear with a luminous contour. On light collecting films having daylight fluorescent properties, such colored luminous effects are also visible in daylight.
According to a further method, a pattern is first printed on the film as it should later appear as a negative image in the metal coating, and the opaque coating, e.g. the metal coating, applied in a second method step. To apply the printed pattern one uses printing inks or varnishes that show poor adhesion on the film and/or on metal coatings, so that the metal coating either alone or together with the printing ink can be removed solely in mechanical fashion by an air or liquid jet. This method is used for making packaging materials and is basically known from German laid open print no. 36 10 379. Instead of a liquid jet, one can also use mechanical scraping means to remove the poorly adhesive ink. It is more advantageous than the above method, however, to use printing inks which can thereafter be dissolved chemically under the metal coating. But it has turned out that the ink is generally not solubilized sufficiently for completely removing the metal layer in the entire printed area. However, if an ultrasonic source is provided in the solution bath or the film is drawn through an ultrasonic bath after the solution bath, the printed film is completely removed in a simple way in one operation.
According to a further method variant, one uses printing inks that become brittle when drying. If the films printed therewith and then metalized are drawn through an ultrasonic bath, these inks are removed without a chemical solution process solely by the effect of the ultrasonic field, thereby producing the desired negative writing in the metal coating. Suitable inks are e.g. inks based on a novolak medium. The ultrasonic bath is in this case a liquid container with an ultrasonic generator disposed therein or thereon, whereby the liquid plays the part of the coupling medium and water can be used in the most simple case.